1. Field of the Invention
The present invention relates to a uniform distribution heat-transfer pipe unit which deals with double-layer fluids along a furnace wall.
2. Description of the Prior Art
In a spiral-wound type furnace variable pressure operation boiler, heat-transfer pipes at the lower part of the furnace extend spirally, whereas at the upper part of the furnace, the heat-transfer pipes extend vertically. The number of the heat-transfer pipes at the upper part is different than that of the pipes at the lower part of the furnace. For this reason, at a junction between the upper and lower parts of the furnace, a branch pipe such as a two-pronged or three-pronged pipe or an intermediate header has been provided to connect the upper and lower pipes.
One example of the prior art will be explained with reference to FIGS. 5 and 6. FIG. 5 shows one example of a spiral-wound type boiler. The boiler comprises a furnace wall pipe unit 10a for connecting a lower collecting header 14 to an upper header 15. Spiral pipes 12 are formed at the lower part of this furnace wall pipe unit 10a, while vertical pipes 11 are formed at the upper part thereof. For this purpose, the middle of the pipe unit 10a is jointed.
FIG. 6 illustrates this joint portion in detail. The number of the upper vertical pipes 11 differs from that of the lower spiral pipes 12, and hence two or three vertical pipes 11 are joined to the single spiral pipe 12. Fins 13 are attached therebetween to keep the furnace airtight.
There arise, however, the following problems inherent in the conventional distribution heat-transfer pipe unit for use with double-layer fluids.
To be specific, the conventional branch (joint) pipe is conceived as a heat-transfer pipe which deals with the double-layer fluids. Therefore, a gas and a liquid are centrifugally separated due to a difference in specific gravity therebetween. Though heat absorption quantities are the same at the upper part (after being branched) of the furnace, temperatures of the pipes occupied mainly by the gases excessively increase, while the pipes occupied mainly by the liquids undergo a smaller rise in temperature by a value equivalent to latent heat. This results in the generation of a large temperature difference therebetween, which may in some cases be a mortal blow to the furnace shaping pipes.
To obviate this problem, there exists an intermediate header system in which a header is interposed between the upper and lower parts of the furnace. Based on this structure, the furnace walls are complicated. This impacts greatly on the manufacturing costs , and makes it difficult to effect a sealing of intra furnace combustion gas.